1. Technical Field
This invention relates to an air bag device for protecting a passenger from an impact developing at the time of a frontal collision of a vehicle, and more particularly to an air bag device in which an bag member can be instantaneously inflated and expanded to effectively restrain the passenger and also to effectively absorb an impact energy acting on the passenger.
2. Background Art
There are known conventional air bag devices that reduce the impact acting on the passenger during a frontal collision by supplementing an effect of a seat belt, if the impact exceeds a predetermined value. In such an air bag device, an bag member is inflated and expanded by high-pressure gas, and the passenger is protected by a cushioning effect of the inflated and expanded bag member. This air bag device is mounted in a central portion of a steering wheel or in an instrument panel. Usually, the bag member used in the air bag device is formed by superposing a reverse-side cloth of a circular shape (on which an inflator for generating gas at the time of a frontal collision is mounted) and a front-side cloth of a circular shape (which abuts against the passenger upon inflation and expansion of the air bag) together, and then sewing peripheral edge portions of the two cloths together.
In the conventional bag member, a plurality of openings (vent holes) are formed in the reverse-side cloth. At the time of a frontal collision of the vehicle, gas generated from the inflator is fed into the interior of the bag member. However, when the pressure within the bag member excessively increases, there is a risk that the passenger strikes against the bag member and rebounds greatly, which is dangerous. Therefore, excess gas is discharged from the interior of the bag member through the vent holes in order to prevent the pressure within the bag member from excessively increasing.
However, the bag member is of the structure in which the openings are merely formed in the reverse-side cloth, and therefore gas is discharged even in the process of inflating and expanding the bag member. Therefore, a large inflator output is required for instantaneously inflating and expanding the bag member. It is also difficult to properly control the pressure within the bag member since the bag member has the structure in which the openings are merely provided.
On the other hand, there is disclosed an air bag device in which a cover is provided to cover a vent hole from the inside in order to properly control a pressure within an bag member (see JP-A-10-71923). In this air bag device, when the pressure within the bag member is low, the vent hole is covered with the cover. When the pressure within the bag member increases, the cover ejects outwardly from the air bag through the vent hole, so that the vent hole is opened. Therefore, the bag member is quickly inflated and expanded at the time of a frontal collision of a vehicle, and the pressure within the bag member can be properly controlled. Therefore the air bag device can effectively restrain the passenger, and can effectively absorb an impact energy.
There is also disclosed an air bag device comprising a first sheet member mounted at the inflator side, a second sheet member disposed at the passenger side, and a third sheet member interconnecting the first and second sheet members. A superposition portion is formed between the first sheet member and the third sheet member, and a gap portion for discharging gas is formed in this superposition portion (See JP-A-2000-43674). In this air bag device, when the pressure of gas generated from an inflator is low, a closed condition is maintained by the superposition portion, a wasteful discharge of the gas will not occur. On the other hand, when the pressure of the gas increases, the superposition portion is moved apart from an bag member by the pressure of the gas, so that the gas is discharged through the gap portion formed in the superposition portion. Therefore, the amount of discharge of the gas as well as the direction of discharge of the gas can be controlled, and the pressure within the bag member can be reduced.